1. Field of the Invention
The present invention relates to an information recording medium having a plurality of information recording layers including information recording layers made of a phase change material.
2. Description of Related Art
DVD-RAM disks are known as information recording media using a phase change material. FIG. 11 shows a track format of a DVD-RAM disk. As shown in the figure, the DVD-RAM disk includes headers 201 called xe2x80x9cCAPAxe2x80x9d (Complementary Allocated Pit Addressing) and lands/grooves 202, in which each land alternates with each groove in the radial direction, extend in the circumferential direction over the disk surface as guide grooves. In each header 201, address information is recorded as pre-pits, but no signals are recorded on a phase change recording film portion extending over the header 201, leaving the phase change recording film portion as-initialized i.e., crystalline. On the other hand, signals are recorded on the lands/grooves 202 in which both an amorphous region (amorphous marks are recorded) and a crystalline region (initialized with no amorphous marks recorded) are present. A recording medium having this land/groove structure for increasing the recording density includes the headers 201 at circumferential locations of the disk surface as described above, and the headers 201 are also used to adjust signals that control the switching of the optical-spot from a land 202A to a groove 202B and vice versa, for example.
Such an information recording medium stacking two or more information recording layers made of a phase change material are disclosed in, for example, EP 0957477 A2 and U.S. Pat. No. 6,221,455 B1.
In the case of an information recording medium having two information recording layers each being formatted analogously to the DVD-RAM track format, for example, a first information recording layer is stacked on a second information recording layer through an optically transparent layer. To record signals on the second information recording layer through the first information recording layer, or to reproduce signals from the second information recording layer through the first information recording layer, there is a difference in the amount of optical transmission between when the laser light transmits through a header of the first information recording layer and when the laser light transmits through a signal-recorded land/groove. Since it is difficult to manufacture a disk with headers such that the headers of the first information recording layer superimpose upon those of the second information recording layer, the optical transmission must be considered for a disk in which the headers of the first information recording layer do not superimpose upon those of the second information recording layer. Then, in such a disk, in accessing the second information recording layer, the laser light transmitting through the first information recording layer fluctuates in accordance with whether signals are recorded or unrecorded on the first information recording layer, i.e., for example, whether the laser light transmits through the header or through the recorded marks on the land/groove, independently of whether information is recorded or not on the second information recording layer. This fluctuation of the laser light in turn results in the fluctuation of recording power during recording on the second information recording layer, and further in the fluctuation of the level of reproduced signals during reproduction from the second information recording layer. When these fluctuations exceed their tolerances, the signals cannot be accurately recorded on the disk/reproduced from the disk.
An object of the invention is, therefore, to provide an information recording medium capable of satisfactorily recording/reproducing information signals on/from the second information recording layer through the first information recording layer having a phase change recording film.
The present invention provides an information recording medium having a light transmissive layer (103), a first information recording layer (L1), a transparent layer (102), and a second information recording layer (L2) sequentially stacked to reproduce information signals upon exposure of the light transmissive layer (103) to laser light, wherein the first information recording layer (L1) is provided with a first protective layer (11), a phase change recording film (1), and a second protective layer (12) sequentially from the light transmissive layer (103), and the rate of power fluctuation between a reproducing light passing through a recorded region of said first information recording layer (L1) and that passing through an unrecorded region of said first information recording layer (L1), during reproduction of the information signals from said second information recording layer (L2), are within 10%.
According to this invention, the rate of fluctuation between a reproducing light passing through a recorded regions of the first information recording layer and that passing through an unrecorded region of the first information recording layer, during reproduction of the information signals from the second information recording layer, are within 10%, whereby optical power fluctuation dependent upon whether signals are recorded or unrecorded on the first information recording layer can be suppressed when signals are recorded/reproduced on/from the second recording layer, and hence information signals can be recorded/reproduced on/from the second recording layer satisfactorily. Additionally, there is no need to adjust the positional relationship between the recorded region of the first information recording layer and that of the second information recording layer.
Assuming that the percentage of an area occupied by recorded marks in the recorded region of the first information recording layer (L1) is Aa, then
0.2xe2x89xa6Aaxe2x89xa60.5
may be satisfied.
In this case, each recorded mark alternates with each unrecorded portion when recorded, and each recorded mark may not be too small, whereby a practically applicable information recording medium can be obtained which is capable of satisfactorily recording/reproducing information signals on/from the second recording layer and which requires no adjustment of the positional relationship between the recorded region of the first information recording layer and that of the second information recording layer.
Assuming that the transmissivity of the recorded region of the first information recording layer (L1) is Tr, then 0.3xe2x89xa6Trxe2x89xa60.8 may be satisfied, and assuming that the transmissivity of the unrecorded region of the first information recording layer (L1) is Tnr, then 0.3xe2x89xa6Tnrxe2x89xa60.8 may be satisfied.
In this case, since 0.3xe2x89xa6Trxe2x89xa60.8 and 0.3xe2x89xa6Tnrxe2x89xa60.8, the transmissivity of the first information recording layer is not too small, whereby sufficiently intense signals can be obtained from the second information recording layer. Furthermore, the transmissivity of the first information recording layer is not too large, either, whereby the amount of light reflecting from the first information recording layer itself and the light absorption coefficient during recording on the first information recording layer can be suppressed. Hence, information signals can be recorded/reproduced satisfactorily.
An information recording medium may be provided which has a light transmissive layer (103), a first information recording layer (L1), a transparent layer (102), and a second information recording layer (L2) sequentially stacked to reproduce information signals upon exposure of the light transmissive layer (103) to laser light, wherein the first information recording layer (L1) is provided with a first protective layer (11), a phase change recording film (1), and a second protective layer (12) sequentially from the light transmissive layer (103), and assuming that the refractive index and extinction coefficient of a crystalline portion of the phase change recording film (1) are n1c and k1c, respectively, and the refractive index and extinction coefficient of an amorphous portion of the phase change recording film (1) are n1a and k1a, respectively, then
(n1cxe2x88x92n1a)xc3x97(k1cxe2x88x92k1a) less than 0 and |(n1c+3/2xc3x97k1c)xe2x88x92(n1a+3/2xc3x97k1a)|xe2x89xa62.
In this case, since the condition (n1cxe2x88x92n1a)xc3x97(k1cxe2x88x92k1a) less than 0 and |(n1c+3/2xc3x97k1c)xe2x88x92(n1a+3/2xc3x97k1a)|xe2x89xa62 is satisfied, optical power fluctuation dependent upon whether signals are recorded or unrecorded on the first information recording layer can be suppressed when signals are recorded/reproduced on/from the second recording layer, and hence information signals can be recorded/reproduced on/from the second recording layer satisfactorily. Additionally, there is no need to adjust the positional relationship between the recorded region of the first information recording layer and that of the second information recording layer.
An information recording medium may be provided which has a light transmissive layer (103), a first information recording layer (L1), a transparent layer (102), and a second information recording layer (L2) sequentially stacked to reproduce information signals upon exposure of the light transmissive layer to laser light, wherein the first information recording layer (L1) is provided with a first protective layer (11), a phase change recording film (1), and a second protective layer (12) sequentially from the light transmissive layer (103), and assuming that the number of protective films constituting the first protective layer (11) is I, the thickness of each of the protective films is di [nm] (where i is a natural number), the refractive index of each of the protective films is ni, the wavelength of the laser light is xcex [nm], and m is an integer, then the sum xcexa3 for all i running from 1 to I is given substantially as
(m/2+1/4)xe2x88x921/8xe2x89xa6xcexa3(di/(xcex/ni))xe2x89xa6(m/2+1/4)+1/8,
and assuming that the number of protective films constituting the second protective layer (12) is J, the thickness of each of the protective films is dj [nm] (where j is a natural number), the refractive index of each of the protective films is nj, and k is an integer, then the sum xcexa3 for all j running from 1 to J is given substantially as (k/2)xe2x88x921/8xe2x89xa6xcexa3(dj/(xcex/nj))xe2x89xa6(k/2)+1/8.
According to this information recording medium, suitable thicknesses are selected for the protective films respectively constituting the first and second protective layers, whereby optical power fluctuation dependent upon whether signals are recorded or unrecorded on the first information recording layer can be suppressed when signals are recorded/reproduced on/from the second recording layer.
An information recording medium may be provided which has a light transmissive layer (103), a first information recording layer (L1), a transparent layer (102), and a second information recording layer (L2) sequentially stacked to reproduce information signals upon exposure of the light transmissive layer (103) to laser light, wherein the first information recording layer (L1) is provided with a first protective layer (11), a phase change recording film (1), and a second protective layer (12) sequentially from the light transmissive layer (103), and assuming that the number of protective films constituting the first protective layer (11) is I, the thickness of each of the protective films is di [nm] (where i is a natural number), the refractive index of each of the protective films is ni, the wavelength of the laser light is xcex [nm], and m is an integer, then the sum xcexa3 for all i running from 1 to I is given substantially as
(m/2+1/4)xe2x88x921/8xe2x89xa6xcexa3(di/(xcex/ni))xe2x89xa6(m/2+1/4)+1/8,
and assuming that the number of protective films constituting the second protective layer (12) is J, the thickness of each of the protective films is dj [nm] (where j is a natural number), the refractive index of each of the protective films is nj, and k is an integer, then the sum xcexa3 for all j running from 1 to J is given substantially as (k/2)xe2x88x921/8xe2x89xa6xcexa3(dj/(xcex/nj))xe2x89xa6(k/2)+1/8, and assuming that the refractive index and extinction coefficient of a crystalline portion of the phase change recording film (1) are n1c and k1c, respectively, and the refractive index and extinction coefficient of an amorphous portion of the phase change recording film (1) are n1a and k1a, respectively, then
(n1cxe2x88x92n1a)xc3x97(k1cxe2x88x92k1a) less than 0 and |(n1c+3/2xc3x97k1c)xe2x88x92(n1a+3/2xc3x97k1a)|xe2x89xa62.
In this case, optical power fluctuation dependent upon whether signals are recorded or unrecorded on the first information recording layer can be suppressed when signals are recorded/reproduced on/from the second recording layer.
While reference symbols referred to in the accompanying drawings are appended to the above description in parentheses to facilitate the understanding of the invention, this is not to limit the invention to embodiments shown in the drawings.